1) Field of the Invention
The present invention relates to a process for forming a thin metal film on a substrate by chemical vapor deposition in the production of semiconductor devices.
2) Prior Art
With higher integration of semiconductor devices the sizes of contact holes or throughholes formed in an insulating film on a substrate have been made extremely finer and finer. It is known that chemical vapor deposition (which will be hereinafter referred to as CVD) is an effective technique of filling or covering a high melting point metal etc. into recesses in a finer pattern, such as contact holes or throughholes in an insulating film on a substrate. In the conventional CVD deposition to form a thin metal film on a substrate by filling or covering the recesses, metal fluorides, for example, fluorides of tungsten, molybdenum, etc. are used as a raw material gas, and are introduced into a CVD chamber as a gas mixture with a reducing gas serving as a reducing agent such as hydrogen (H.sub.2) gas or a silane (SiH.sub.4) gas, thereby forming a thin metal film thereon.
However, the formation of a thin metal film on a substrate by CVD substantially depends on selected raw material gas and reducing gas and an etching reaction of a substrate wafer or deposited thin metal film takes place, depending on a reaction process of the selected raw material gas and reducing gas. For example, in the formation of a thin tungsten film on a substrate, reduction reaction to reduce tungsten hexafluoride to a metallic tungsten by hydrogen or silanes is most frequently used to form the metallic tungsten as ultimate products of the reactions. Thus, when a hydrogen gas is used as a reducing gas, the necessary reaction temperature is 400.degree. to 500.degree. C., at which etching reaction of silicon wafer is promoted, or the surface of the deposited thin metal film is roughened. When silanes are used as reducing gases, silicon is highly liable to retain in the deposited metal film, deteriorating the quility of the thin metal film, though the reduction reaction can proceed at a lower temperature. That is, when a hydrogen gas is used as a reducing gas, a high reduction temperature (high substrate temperature) is required for the reaction, resulting in roughening of the deposited thin metal film or etching of substrate wafer, which leads to current leakage or peeling of the thin metal film. When silanes are used as reducing gases, the deposited thin metal film is contaminated with silicon, resulting in higher resistivity and residual strain of the deposited thin metal film.